With continued growth in demand for electronic communications of all kinds, and with spectrum being a finite resource, interest has arisen in finding techniques for improving the spectral efficiency of point-to-point, line-of-sight MMW communications. One such technique is to increase the constellation size of the modem, see J. G. Proakis, Digital Communications, 3rd Edition, McGraw-Hill International Editions, 1995; however, there is a practical limit to the size of the constellation that can be used in MMW communications, and constellations higher than 256 have been found to be impractical. Another technique is to utilize multiple antenna systems in combination with some mechanism to create channel discrimination, as described for example in I. E. Telatar, “Capacity of multi-antenna Gaussian channels,” Eur. Trans. Telecom., vol. 10, pp 585-595, November 1999, in G. J. Foschini, “Layered space-time architecture for wireless communication in fading environment when using multi-element antennas,” Bell Labs Tech. J., vol. 1, no. 2, pp. 41-59, 1996, and in N. Amitay and J. Salz, “Linear equalization theory in digital data transmission over dually polarized fading radio channels,” AT&T Tech. J., vol. 63, no. 10, Part 1, pp. 2215-2259, December 1984. One example in Telatar above describes the use of multiple antennas in microwave systems, where the channel discrimination is achieved by differential fading effects that each of the radio paths experience. Another example is the use of multiple antennas in MMW communication, described generally in PCTJ/S01/24913 filed Aug. 9, 2001, and PCT/IL01/00820 filed Aug. 30, 2001, where channel discrimination is achieved by the geometric structure of the arrays. However, both Telatar and the two PCT applications above require a (relatively) large physical separation between the antennas, depending on the link length and carrier frequency. Separated antennas require separate transceivers, which result in increased phase noise effects. Separate antennas may not be practical at a particular location due to zoning or physical mounting constraints. In addition, in such systems each antenna at the receiver receives signals from all transmitting antennas, with approximately the same power, requiring sophisticated equalization and decoding techniques.
One promising technique for increasing MMW spectral efficiency is dual polarity transmission, described in Amitay above. Dual polarity transmission is a form of a multiple antenna system, where two antennas are used at each side of the point-to-point link. In this case channel discrimination is achieved by transmitting at orthogonal polarizations, commonly known as vertical (V) and horizontal (H) polarizations. In such a system, the V and H antennas can be combined into a single physical antenna known as a dual-polarized antenna, which enables the system to utilize part of the same radio elements for the two transmissions.
Dual polarity transmission systems have successfully been used in lower-frequency microwave radio systems to improve spectral efficiency. However, the lower frequencies are not affected by rain fade, which is the primary cause of fading in MMW channels. Rain attenuates the H polarization more than the V polarization, a phenomenon due to the flattening (lengthening in the H plane) of raindrops as they fall to earth. Existing dual polarity coding systems created for microwave links code separately for the two polarities, meaning the weaker polarity (H) dictates the link performance (achievable link capacity). In addition, the reflection of electromagnetic waves from a surface depends on the polarization, thus in some scenarios the two polarizations can experience different multipath effects. There is thus a need for a coding system that combines (in some way) the two polarities, and that can therefore exploit the diversity of the channels in favor of link capacity. Asymmetric modulations and joint interleaving and coding exploit the diversity between the channels and increase the overall transmission rate significantly.